The present invention relates to a two component developing agent containing a non-chromium type charge controlling agent and used in an image-forming apparatus of an electrophotographic system and also relates to a developing device using the same.
In an electrophotographic apparatus or an electrostatic recording apparatus, an electrostatic latent image formed on an electrostatic image holder made of a photoreceptor or a dielectric body is made visible by development with a developing agent of a two component developing system using a toner and a carrier or with a developing agent of a one component system using a toner which also acts as a carrier.
As disclosed in, for example, Japanese Patent disclosure (Kokai) No. 5-313404, a method of preparing a toner comprises in general the step of fusing a mixture of, for example, a thermoplastic resin, a dye, a pigment and other additives such as wax so as to uniformly disperse these components, followed by solidifying under cooling the fused mixture. The solidified mixture is pulverized and, then, classified to obtain colored fine toner particles of a desired particle size.
The dye contained in the developing agent also plays an important role as a charge controlling agent for controlling the electrostatic charging. As disclosed in, for example, Japanese Patent Disclosures Nos. 57-141452 and 58-11049, dyes containing complex compounds of heavy metals, e.g., chromium-containing complex compounds, are widely used as, for example, dyes for electrostatic negative charging. However, heavy metals such as chromium are harmful to human bodies. Therefore, it is of high importance to develop a charge controlling agent which does not contain a heavy metal in view of safety and pollution problem.
Recently, dyes containing iron complex compounds, which are markedly advantageous in safety in the manufacturing process, are disclosed in, for example, Japanese Patent Publication (Kokoku) No. 4-75263.
However, in the case of using a dye containing iron complex compounds which are advantageous in safety, the developing agent is electrostatically charged moderately, compared with the conventional developing agent containing a dye containing heavy metals, leading to a toner dusting problem in, particularly, a high speed developing process.
It should also be noted that the picture image quality is deeply related to the particle diameters of the carrier and toner. Specifically, the image quality can be improved with decrease in the particle diameters of the carrier and toner. However, if these particle diameters are made excessively small, the flowability of the developing agent is impaired, giving rise to problems such as dusting of the toner and thinning in the latter part of the black solid print in the printing test.